Ive got to many favortes that never top each other. right now im listening to 8 days a week by the beatles. some more favorties is freinds by LZ and my michelle by GNR.
It may be the song 'Tir Nan Og' by the band Alcest, which is a neofolk acoustic song by a black metal/shoegaze(!) band from France. It's a very dreamlike atmospheric song, and it has strangely enough both an uplifting serenity and a melancholic sense of yearning to it. All six songs from this album (entitled Souvenirs d'un Autre Monde - all older releases are more black metal orientated) share this atmosphere, but this is the only song without electric guitars. It's absolutely beautiful from beginning to end. You don't have to be a fan of black metal to like this album, as it's very melodic, very clean in production, and there is no aggression behind it whatsoever. So... you might wanna check it out if you're into haunting atmospheric music of any kind!
right now "its dancing behind my eyelids" by Múm off their album Go Go Smear the Poison Ivy that comes out in a month.
http://youtube.com/watch?v=ike25aQ8rfs Don't mind the shitty video, but the song gets me into space everytime I listen to it.